66 Remarks on the Nomenclature of Achromatic 
act sluggishly, or if the excitability of these nerve fibres is numbed by 
certain agents, time is allowed for the poison just outside the vessels 
to make its way into the blood ; while if they are paralyzed, even 
temporarily, the state of things is far worse. No watch is kept, 
and the organism may be destroyed by an invading poison that 
under other circumstances might have been effectually excluded. 
III . — Remarks on the Nomenclature of Achromatic Objectives for 
the Compound Microscope. 
By Dr. J. J. Woodward, U. S. Army. 
For some years past, while most of the Continental opticians have 
continued to give arbitrary designations to their achromatic object- 
glasses, such as No. 1, No. 2, &c., or System A, System B, &c., 
the English and American manufacturers, affecting a higher degree 
of accuracy, have undertaken to name the objectives they construct 
by their real or supposed agreement in magnifying power, with 
single lenses of specified focal lengths. We hear accordingly of 
inch, half-inch, and quarter-inch objectives, &c., by which we are 
expected to understand combinations agreeing in magnifying power 
wfith single convex lenses of the focal lengths named. 
At first sight nothing could appear simpler or more exact than 
such a nomenclature; nevertheless recent articles in the journals 
would seem to indicate that the general plan is capable of con- 
siderable modification in its practical application, and that grave 
misunderstandings have hence arisen. 
Under these circumstances, it appears desirable to give some 
account of the principles involved, and of the practical difficulties 
to be considered in their application, particularly as the micro- 
scopical text-books contain little or no information on the subject. 
In fact, the only scientific discussion of the matter with which I 
am acquainted is the paper of Mr. Charles R. Cross, “ On the Focal 
Length of Microscopic Objectives.”* This paper gives a reasonable 
formula for the approximate computation of equivalent focal lengths, 
and furnishes some other valuable information, but does not discuss 
all the points at issue. I shall have occasion to refer more than 
once to this excellent paper, which the reader would do well to 
examine in connection with the following remarks. 
We learn from the elementary treatises on optics that when an 
object is placed in front of a single convex lens at a distance some- 
what greater than its focus for parallel rays, a real image is formed 
* ‘Journal of tlio Franklin Institute,’ June, 1S70, p. 401. 
